Cytological features of the neuron
Nerve cells are distinguished by a high metabolic rate. The neuronal nucleus is typically pale: euchromatin (decondensed chromatin) predominates, indicating active transcription processes. The synthetic apparatus is represented by a well-developed rough endoplasmic reticulum (rER), a prominent Golgi apparatus, numerous mitochondria, and lysosomes. Such a robust apparatus is necessary for the continuous synthesis of RNA and proteins to maintain function and support the regeneration of long processes.
The cytoplasm (perikaryon) of the neuron contains the following specific structures:
- Basophilic substance (tigroid substance, Nissl bodies). These are accumulations of rER cisterns. It is readily revealed by basic stains due to the abundance of RNA in ribosomes. Tigroid substance ensures the synthesis of export, membrane, and lysosomal proteins. It is present in the cell body and dendrites, but is completely absent in the axon and axon hillock.
- Neurofibrils. Cytoskeletal elements (bundles of neurofilaments and neurotubules). Upon silver impregnation, they form a dense three-dimensional network in the cell body and run in parallel bundles within the processes. Their appearance in embryogenesis is the first sign of neuroblast differentiation.
- Lipofuscin. A brown pigment that accumulates as the cell ages (it can occupy up to 50% of the cytoplasmic volume in advanced age).
- Neurosecretory granules. Characteristic of hypothalamic secretory neurons. They are membrane-bound vesicles containing peptides, transported along the axon to blood vessels.
Processes: axons and dendrites
The main classifying criterion for processes in the Russian histological school is the direction of impulse conduction, rather than their length or myelination.
Dendrites Conduct nerve impulses afferently, i.e., toward the neuron cell body. There are usually multiple dendrites, and they branch close to the cell body. The surface of dendrites features specialized outgrowths—dendritic spines, which are necessary for synapse formation. Inside the spines are cisterns of smooth ER (calcium ion stores). The higher the cellular activity, the more spines on its dendrites.
Axon (neurite) Conducts impulses efferently—away from the neuron cell body. There is always only one axon. It typically does not branch along its course, giving off collaterals only in its terminal part. It originates from a specialized region of the perikaryon—the axon hillock, where nerve impulse generation occurs.
Morphological classification of neurons
Based on the number of processes extending from the cell body:
- Unipolar. Have only one axon. In humans, they are found only during embryogenesis (neuroblasts).
- Pseudounipolar. The points of origin of the dendrite and axon are extremely close together: it appears as if a single trunk emerges from the cell body and then branches in a T-shape. These are sensory neurons; their cell bodies are always located outside the CNS—in spinal ganglia. A long dendrite extends peripherally to receptors, while a short axon extends into the spinal cord.
- Bipolar. Have two processes (a dendrite and an axon) extending from opposite poles. A rare type, found in sensory organs (retina, olfactory epithelium).
- Multipolar. The most common type. Have one long axon and multiple branching dendrites. Functionally, they are associative or effector.
Functional classification and impulse conduction
A neuron processes information in 4 stages: reception (signal perception), state change (excitation/inhibition), conduction, and transmission. The excitation mechanism is based on plasmalemma transport systems. In the resting state, the membrane is polarized. Upon depolarization, sodium channels open, sodium rushes into the cell, and the charge decreases. Inhibition (hyperpolarization) is provided by the opening of anion channels.
Functionally, neurons are divided into:
- Sensory (afferent): perceive impulses from receptors. Cell bodies lie exclusively in ganglia.
- Associative (interneurons): form connections between cells within the CNS or autonomic ganglia.
- Effector (motor/secretory): transmit commands to effector organs (muscles, glands).
Signal transmission to other cells occurs primarily via synapses using chemical neurotransmitters, or via the bloodstream (neurosecretory transmission via axovascular contacts).
Briefly: answers to topical questions
Which neuroglial cells form the myelin sheath of axons in the central and peripheral nervous systems?
The myelin sheath of nerve fibers is formed by specialized glial cells that wrap around the axon (axis cylinder). The cellular composition of the sheath strictly depends on the division of the nervous system:
| Division of the nervous system | Glial cells |
|---|---|
| Central nervous system | Oligodendrocytes |
| Peripheral nervous system | Lemmocytes (neurolemmocytes, Schwann cells) |
The myelin layer itself consists of multiple layers of the glial cell plasma membrane (oligodendrocyte or lemmocyte). Sheath formation occurs by the immersion of the neuron process into the cell cytoplasm followed by multiple rotations of the lemmocyte around its axis, resulting in concentric wrapping of the membranes.
What is the mechanism of axonal transport and which proteins are involved in it?
The mechanism of axonal transport directly depends on the physical state of the transported substance and is carried out using neurotubules. Rapid transport of dissolved substances occurs as a fluid current driven by hydrodynamic pressure through the intertubular space. The transport of vesicles, granules, and organelles requires ATP energy and operates on a "monorail" principle: a motor protein attaches to the cargo with one part and to a neurotubule with the other, performing stepping movements.
The following motor proteins are involved in the transport of formed elements:
- Kinesin — provides forward (anterograde) transport.
- Dynein — provides reverse (retrograde), centripetal transport.
To avoid confusing the processes: Dendrite — from the word "Tree" (dendro-), it branches and directs juices (impulses) toward the "roots" (the cell body). Axon — starts with the letter "A" (away), there is always only one, and it conducts impulses away from the cell body.
Class and exam questions
How to distinguish an axon from a dendrite in a tissue preparation stained with Nissl stain?
In the axon and its site of origin (axon hillock), the basophilic substance (Nissl bodies) is completely absent. In dendrites and the perikaryon itself, the tigroid substance is clearly visualized.
What is the difference between saltatory and continuous impulse propagation?
In continuous conduction, the wave of depolarization runs along the entire length of the plasmalemma. In saltatory (jumping) conduction, sodium channels are concentrated only in specific regions isolated by myelin, allowing the signal to "jump" between them faster.
Where are the cell bodies of sensory pseudounipolar neurons located?
Their cell bodies are always located outside the central nervous system—they are localized in sensory ganglia (e.g., in spinal ganglia).
What else to study on this topic
- Mechanisms of continuous and saltatory nerve impulse conduction
- Ultrastructure of synapses and mechanisms of synaptic transmission
- Structure of peripheral nerve fibers and the myelin sheath
- Morphology of the spinal ganglion and satellite glial cells
- Neurosecretory function of the hypothalamus and axovascular contacts
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